The present invention relates to a liquid crystal display device; and, more specifically, the invention relates to an in-plane field type liquid crystal display device.
In general, an in-plane field type liquid crystal display device is provided with a pixel electrode and a counter electrode spaced apart from the pixel electrode in each pixel area on a liquid-crystal-side surface of one of the transparent substrates which are opposed to each other via a liquid crystal, and the optical transmittance of the liquid crystal is controlled by means of an electric field which is selectively generated between the pixel electrode and the counter electrode.
A liquid crystal display device of an active matrix type which adopts the in-plane field mode of operation includes, on a liquid-crystal-side surface of one of its transparent substrates, scanning signal lines extended in the x direction and juxtaposed in the y direction, counter signal lines extended in the x direction and juxtaposed in the y direction, and video signal lines extended in the y direction and juxtaposed in the x direction, and each area surrounded by adjacent scanning and counter signal lines and a pair of adjacent video signal lines is formed as a pixel area. In each pixel area, there is a thin-film transistor driven by the supply of a scanning signal from the scanning signal line and a pixel electrode to which a video signal is supplied from one of the video signal lines via the thin-film transistor. In this case, since a voltage corresponding to the video signal relative to the counter electrodes is applied to the pixel electrode, a reference voltage is applied to the counter electrode via the counter signal line.
In such a liquid crystal display device, since its structure is such that the thin-film transistors are easily damaged by static electricity, it is indispensable to take measures against static electricity. As an attempt at taking measures against static electricity, there has been a proposal in which a protection element common line is formed to surround a display area formed by an assembly of pixels, and scanning signal lines and video signal lines are connected to the protection element common line via non-linear elements at the intersections of the scanning and video signal lines and the protection element common line.
For example, a technique for taking measures against static electricity in a liquid crystal display device, which differs in structure from the in-plane field mode, is described in Japanese Patent Laid-Open No. 9-90428. In accordance with this technique, as measures against static electricity, scanning signal lines and video signal lines are connected to a common line via static protection elements. Although the static protection elements are formed of high-resistance elements so that static electricity flows through the static protection elements, the scanning signal lines and the video signal lines are electrically connected via the static protection elements and the common line. For this reason, it has been found, when the liquid crystal display device is being driven by an external circuit, part of the video signals from the video signal lines flow into the static protection elements as leakage current and this leakage current flows into the counter electrodes of the respective pixels via the common line, creating problems in that the image quality is degraded and the voltage at the video signal lines is lowered.